The purpose of the study is to determine the influence of HIV virus load, characteristics of the virus and the presence of symptoms on response to new therapeutic regimens and on the development of viral resistance mutation in HIV infected individuals who have been treated with monotherapy with either ZDV or ddI on a previous ACTG trial (ACTG 175). More specifically, this study will determine whether the addition of 3TC or a change to d4T in patients who have been on ZDV result in a more rapid or durable suppression of plasma viral RNA.